Substrate design with balanced metal and solder resist density

ABSTRACT

A package includes a package substrate, which includes a middle layer selected from the group consisting of a core and a middle metal layer, a top metal layer overlying the middle layer, and a bottom metal layer underlying the middle layer. All metal layers overlying the middle layer have a first total metal density that is equal to a sum of all densities of all metal layers over the middle layer. All metal layers underlying the middle layer have a second total metal density that is equal to a sum of all densities of all metal layers under the middle layer. An absolute value of a difference between the first total metal density and the second total metal density is lower than about 0.1.

PRIORITY CLAIM AND CROSS-REFERENCE

This application is a continuation of U.S. patent application Ser. No.14/080,106, entitled “Substrate Design with Balanced Metal and SolderResist Density,” filed on Nov. 14, 2013, now U.S. Pat. No. 9,153,550 B2,which application is incorporated herein by reference.

BACKGROUND

Bump-on-Trace (BOT) structures are used in flip chip packages, whereinmetal bumps are bonded onto narrow metal traces in package substratesdirectly, rather than bonded onto metal pads that have larger sizes thanthe respective connecting metal traces. The BOT structures requiresmaller chip areas, and the manufacturing cost of the BOT structures islow. The conventional BOT structures may achieve the same reliability asthe conventional bond structures that are based on metal pads. In atypical BOT structure, a solder region is formed on a surface of acopper bump of a device die. The solder region bonds the copper bump toa metal trace in a package substrate. The solder region contacts a topsurface and sidewalls of the metal trace, hence forming the BOTstructure.

Since the existing BOT structures have very small spacings, bridging mayoccur, wherein the solder region of one BOT bond structure is bridged toa neighboring metal trace. Particularly, the BOT structures in theperipheral areas of the packages are more likely to bridge due to thehigh density of the BOT structures in the peripheral areas. In addition,in the peripheral areas, the distance of the BOT structures are fartheraway from the centers of the respective packages. Accordingly, duringthe reflow process for forming the BOT structures, the shift of the BOTstructures caused by the thermal expansion of the metal traces is moresignificant than in the areas close to the centers of the respectivepackages. Accordingly, the bridging is more likely to occur. The shiftof the BOT structures may be caused by warpage of the package substrate.In addition, in the BOT structures, metal traces are thin, and thevolume of the solder used in the BOT structures is small. When thewarpage occurs, poor solder join may also occur. Both solder bridgingand poor solder joint cause the drop of the assembly yield in thepackaging process.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the embodiments, and the advantagesthereof, reference is now made to the following descriptions taken inconjunction with the accompanying drawings, in which:

FIG. 1 illustrates a cross-sectional view of a package substrate inaccordance with some exemplary embodiments, wherein the packagesubstrate includes a core;

FIG. 2 illustrates the bonding of a package component to the packagesubstrate through Bump-On-Trace (BOT) bonding in accordance with somealternative exemplary embodiments; and

FIG. 3 illustrates the bonding of a package component to a corelesspackage substrate through BOT bonding in accordance with somealternative exemplary embodiments.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the embodiments of the disclosure are discussedin detail below. It should be appreciated, however, that the embodimentsprovide many applicable concepts that can be embodied in a wide varietyof specific contexts. The specific embodiments discussed areillustrative, and do not limit the scope of the disclosure.

A package substrate and a package Bump-On-Trace (BOT) bonding areprovided in accordance with various exemplary embodiments. Thevariations of the embodiments are discussed. Throughout the variousviews and illustrative embodiments, like reference numbers are used todesignate like elements.

FIG. 1 illustrates a cross-sectional view of package substrate 20.Package substrate 20 may include core 24, and metal layers formed on theopposite sides of core 24. In accordance with some embodiments, thenumber of metal layers over core 24 is equal to the number of metallayers under core 24. Throughout the description, the term “metal layer”refers to the collection of all metal features, including, and notlimited to, metal traces and metal pads, that are at the same level. Thecorresponding parts/layers of the metal features in the same metal layerare formed of the same materials that have the same compositions. Forexample, all metal features in a metal layer may include copper, or acopper layer and a nickel layer over the copper layer. In some exemplaryembodiments as shown in FIG. 1, package substrate 20 includes metallayers L2 overlying core 24, and metal layer L1 over metal layer L2.Package substrate 20 further includes metal layer L3 underlying core 24,and metal layer L4 underlying metal layer L3.

Metal layers L1, L2, L3, and L4 are electrically interconnected throughmetal vias 25, 26, and 28. Accordingly, metal traces 30, which are partsof metal layer L1, are electrically connected to metal pads 32, whichare parts of metal layer L4. The metal features in metal layers L1, L2,L3, and L4 may comprise copper, aluminum, nickel, gold, or combinationsthereof. Core 24 includes core dielectric layer 27, and metal vias 28penetrating through core dielectric layer 27. In some embodiments, coredielectric layer 27 comprises one or more material selected from epoxy,resin, glass fiber, molding compound, plastic (such as PolyVinylChloride(PVC), Acrylonitril, Butadiene & Styrene (ABS), Polypropylene (PP),Polyethylene (PE), PolyStyrene (PS), Polymethyl Methacrylate (PMMA),Polyethylene Terephthalate (PET), Polycarbonates (PC), Polyphenylenesulfide (PPS), combinations thereof, and multi-layers thereof. Metalvias 28 may be formed as conductive pipes in some exemplary embodiments.The internal volumes of metal vias 28 are filled with dielectric filling29, which may be a material selected from the same candidate materialsfor forming core dielectric layer 27. In alternative embodiments,conductive pipes 28 comprise air gaps therein. Metal vias 28electrically interconnect, and may be in physical contact with, themetal features in metal layers L2 and L3.

Dielectric layer 38 is formed over core 24, with the vias 26 thatinterconnect metal layers L1 and L2 disposed in dielectric layer 38.Dielectric layer 38 may be formed of PP in some exemplary embodiments,while other dielectric materials may also be used. In some embodiments,dielectric layer 38 comprises a planar top surface 38A, wherein thebottom surface of metal layer L1 are over and in contact with topsurface 38A. Furthermore, dielectric layer 42 is formed over dielectriclayer 38. Dielectric layer 42 may have the lower portions level withmetal layer L1, and may, or may not, include upper portions higher thanmetal layer L1. In some embodiments, dielectric layer 42 has thicknessT1 that is greater than thickness T2 of metal layer L1. Dielectric layer42 may cover some portions of dielectric layer 38, while leaving otherportions of dielectric layer 38 exposed. In some embodiments, dielectriclayer 42 also covers some parts of metal layer L1. For example, as shownin FIG. 1, dielectric layer 42 covers some parts of metal traces 30.

In some embodiments, dielectric layers 38 and 42 are formed of differentdielectric materials. In alternative embodiments, dielectric layers 38and 42 are formed of a same dielectric material. Dielectric layer 42 maybe formed of a solder resist, and hence is referred to as solder resist42 hereinafter, although it may also be formed of other dielectricmaterials. Solder resist 42 is a thin lacquer-like layer of polymer.Solder resist 42 may comprise an epoxy resin, a hardener, a color, afiller, and a solvent. For example, the epoxy resin may be o-Cresolnovolac epoxy resin, phenol epoxy resin, or Diglycidylether Bisphenol A(DGEBA) epoxy resin. The active ingredient of the hardener may includethe reaction product of an amine compound such as an imidazole ormelamine. The filler may comprise silicon, aluminum, magnesium, calcium,titanium, or the like. The solvent may comprise glycol ether or thelike.

Dielectric layer 40 is formed under core 24 and dielectric layer 40,with the vias 25 that interconnect metal layers L3 and L4 disposed indielectric layer 40. Dielectric layer 40 may be formed of PP in someexemplary embodiments, while other dielectric materials may also beused. In some embodiments, dielectric layer 40 comprises a planar bottomsurface 40A, wherein the top surface of metal layer L4 is over and incontact with bottom surface 40A.

Dielectric layer 44 is formed underlying, and in contact with,dielectric layer 40. Dielectric layer 44 may have the lower portionslevel with metal layer L4, and may, or may not, include lower portionslower than metal layer L4. In some embodiments, dielectric layer 44 hasthickness T3 that is greater than thickness T4 of metal layer L4. Insome embodiments, some portions of dielectric layer 44 are overlapped bysome parts of metal layer L4. For example, as shown in FIG. 1,dielectric layer 44 is overlapped by some edge parts of metal pads 32.

In some embodiments, dielectric layers 40 and 44 are formed of differentdielectric materials. In alternative embodiments, dielectric layers 40and 44 are formed of a same dielectric material. Dielectric layer 44 maybe formed of a solder resist, and hence is referred to as solder resist44 hereinafter, although it may also be formed of other dielectricmaterials, which may be a polymer.

Metal layer L1 has density DL1, which is calculated as the ratio of thetotal top-view area of all metal features in metal layer L1 to the totaltop-view area of package substrate 20. Similarly, metal layer L2 hasdensity DL2, which is calculated as the ratio of the total top-view areaof all metal features in metal layer L2 to the total top-view area ofpackage substrate 20. Metal layer L3 has density DL3, which iscalculated as the ratio of the total top-view area of all metal featuresin metal layer L3 to the total top-view area of package substrate 20.Metal layer L4 has density DL4, which is calculated as the ratio of thetotal top-view area of all metal features in metal layer L4 to the totaltop-view area of package substrate 20. All of the metal layers overlyingcore 24 have a first total metal density. For example, in the structureshown in FIG. 1, the first total metal density is (DL1+DL2). All of themetal layers under core 24 have a second total metal density. Forexample, in the structure shown in FIG. 1, the second total metaldensity is (DL3+DL4). In some embodiments, a metal density differenceΔDL is defined as being the absolute value of the difference between thefirst total metal density and the second metal density. For example, inthe illustrated embodiments, metal density difference ΔDL is|(DL1+DL2)−(DL3+DL4)|.

In the embodiments in FIG. 1, there are two metal layers over core 24and two metal layers underlying core 24. It is appreciated that apackage substrate may have different numbers of metal layers thanillustrated. For example, a package substrate may include a single metallayer L1′ (not shown) over the core and a single metal layer L2′ (notshown) under the core. Accordingly, metal density difference ΔDL is|DL1′−DL2′|, wherein DL1′ is the metal density of metal layer L1′, andDL2′ is the metal density of metal layer L2′. Alternatively, a packagesubstrate may include three or more layers over the core, and three ormore layers under the core. In these embodiments, the metal densitydifference ΔDL remains to be the absolute value of the total metaldensity of all metal layers over the core minus the total metal densityof all metal layers under the core.

Metal layers L1 through L4 have Coefficient of Thermal Expansions (CTE)greater than the CTE of core 24. For example, the CTE of metal layers M1through M4 may be about 16.5 ppm when formed of copper. The CTE of core24 may be about 10 ppm. When temperature changes, the metal layers overcore 24 has an effect opposite to the effect of the metal layers undercore 24, which effect, when not balanced and are not fully cancelledeach other, may cause the warpage of package substrate 20 when heated orcooled. For example, when heated, the metal layers overlying core 24tend to cause warpage of package substrate 20 so that the edges ofpackage substrate are lower than the center of package substrate.Conversely, when heated, the metal layers underlying core 24 tend tocause warpage of package substrate 20 so that the edges of packagesubstrate are higher than the center of package substrate. The effect ofa metal layer to the warpage increases with the increase in the densityof the respective metal layer. Accordingly, to balance and cancel outthe effects of all metal layers (such as metal layers L1 through L4),metal density difference ΔDL is designed to be smaller than about 10percent (which is 0.1) in order to control the warpage of packagesubstrate 20. The warpage directly affects the assembly yield of thepackage substrate 20, and the higher the warpage is, the lower theassembly yield will be. Experiments results indicated that when metaldensity difference ΔDL is smaller than about 10 percent, the assemblyyield of the package substrates is in an accepted specification that isgenerally accepted by the packaging industry, while a metal densitydifference ΔDL greater than 10 percent results in the assembly yield ofthe respective package substrate to be out of the specification.

Solder resist 42 has density DSR1, which is calculated as the ratio ofthe total top-view area of solder resist 42 to the total top-view areaof package substrate 20. Solder resist 44 has density DSR2, which iscalculated as the ratio of the total top-view area of solder resist 44to the total top-view area of package substrate 20. Similarly, solderresists 42 and 44 have high CTEs (for example, about 60 ppm), which ismuch higher than the CTE of core 24 (for example, about 10 ppm). Thedifference between the CTE of solder resists 42/44 and core 24 may alsocause the warpage of package substrate 20 when package substrate 20 isheated or cooled. In some embodiments, a solder resist densitydifference ΔDSR is defined as being the absolute value of the differencebetween the density DSR1 of solder resist 42 and the density DSR2 ofsolder resist 42. For example, in the illustrated embodiments, solderresist density difference ΔDSR is |DSR1−DSR2|.

Since solder resists 42 and 44 are on the opposite sides of core 24,they have opposite effects on the warpage of package substrate 20 also.The effect of solder resists 42 and 44 to the warpage increases with theincrease in the density of the respective one of solder resists 42 and44. Accordingly, to balance the effects of solder resists 42 and 44,solder resist density difference ΔDSR is designed to be smaller thanabout 50 percent (which is 0.5). Experiments results indicated that whensolder resist density difference ΔDSR is smaller than about 50 percent,the assembly yield of package substrate will be in an acceptedspecification that is generally accepted by the packaging industry,while a solder resist density difference ΔDSR greater than 50 percentresults in the warpage of package substrate to be out of thespecification. Solder resist density difference ΔDSR may also be smallerthan about 45 percent (0.45) to leave an acceptable margin so that theassembly yield may be reliably in the specification.

FIG. 2 illustrates a cross-sectional view of the bonding of device die46 onto package substrate 20. In accordance with some embodiments,device die 46 includes active devices (not shown) such as transistors,passive devices (not shown) such as capacitors, resistors, inductors, orthe like. Non-solder metal bumps 48 are formed at the surface of devicedie 46, wherein metal bumps 48 may include copper pillars, and mayinclude one or more layers comprising nickel, gold, palladium, and/orthe like. Solder regions 50 bond metal bumps 48 to metal traces 30. Thebonding is through a Bump-On-Trace (BOT) method, wherein solder regions50 are in contact with the top surface and the opposite sidewalls ofmetal traces 30. Different from the bonding schemes in which large metalpads are formed for bonding, metal traces 30 may have a substantiallyuniform width, with the un-bonded portions that are not in contact withsolder and the bonded portions that are in contact with solder having auniform width (or a substantially uniform width).

FIG. 2 also illustrate solder balls 52 on metal pads 32. Hence, solderballs 52 are attached to metal layer L4 in accordance with theillustrated embodiments. Solder balls 52 are thus electrically coupledto metal bumps 48 through metal layers L1 through L4.

In FIGS. 1 and 2, package substrates 20 have cores 24. FIG. 3illustrates a coreless package substrate 20, which does not include acore. Instead, a middle metal layer L5 is formed between layers L1/L2and layers L3/L4. In these embodiments, the metal layer density of themiddle metal layer L5 is not counted, and hence metal density differenceΔDL remains to be |(DL1+DL2)−(DL3+DL4)|, wherein metal layer densitiesDL1, DL2, DL3, and DL4 are calculated essentially the same as in theembodiments shown in FIGS. 1 and 2. For these embodiments, metal densitydifference ΔDL is also selected to be smaller than about 10 percent(0.1). Furthermore, solder resist density difference ΔDSR is also|DSR1−DSR2|, and is selected to be smaller than about 50 percent (0.5),wherein densities DSR1 and DSR2 are the densities of solder resists 42and 44, respectively. Solder resist density difference ΔDSR may also besmaller than about 45 percent (0.45).

The BOT bond structures, due to the thin metal traces 30 and smallamount of solder in solder regions 50, are more prone to the bondfailure due to the warpage of package substrate 20. For example, packagesubstrates with the warpage equal to 12 μm, 17 μm, 21 μm-30 μm, and 27μm-40 μm may have the joint yield of 98.5%, 96.5%, 90.0, and 61.0%,respectively. This indicates that the assembly yield has a directrelationship with the warpage of the package substrates. Accordingly, toimprove the assembly yield, the warpage of the package substrate needsto be reduced, and hence the solder resist density difference ΔDSR andmetal density difference ΔDL are controlled in the embodiments of thepresent disclosure.

The embodiments of the present disclosure have some advantageousfeatures. In the embodiments of the present disclosure, by reducing thedifference in the densities of the metal layers that are on oppositesides of the middle line of the package substrates, the warpage of thepackage substrates are reduced, and the assembly yield is increasedsignificantly. Experiments are performed to form sample packagesubstrates and to perform BOT bonding on the sample package substrates.In a first group of sample package substrates, metal layer densitydifference ΔDL is 10.40%, and solder resist density difference ΔDSR is53.3%. In the second group of sample package substrates, metal layerdensity difference ΔDL is 6.42%, and solder resist density differenceΔDSR is 40.46%. In the third group of sample package substrates, metallayer density difference ΔDL is 8.25%, and solder resist densitydifference ΔDSR is 39.39%. The assembly yield values of the first, thesecond, and the third groups of package substrates in the BOT bondingare smaller than 90 percent (the industry accepted specification),higher than 96 percent, and higher than 98 percent, respectively.Accordingly, the first group of sample package substrates has anassembly yield lower than the generally acceptable assembly yield of thepackaging industry, while the second and the third groups of samplepackage substrate have assembly yields higher than the generallyacceptable assembly yield.

In accordance with some embodiments, a package includes a packagesubstrate, which includes a middle layer selected from the groupconsisting of a core and a middle metal layer, a top metal layeroverlying the middle layer, and a bottom metal layer underlying themiddle layer. All metal layers overlying the middle layer have a firsttotal metal density that is equal to a sum of all densities of all metallayers over the middle layer. All metal layers underlying the middlelayer have a second total metal density that is equal to a sum of alldensities of all metal layers under the middle layer. An absolute valueof a difference between the first total metal density and the secondtotal metal density is lower than about 0.1.

In accordance with other embodiments, a package includes a packagesubstrate, which includes a core, a top metal layer overlying the core,and a bottom metal layer underlying the core. A first solder resistincludes a portion level with the top metal layer, wherein the firstsolder resist has a first solder resist density. A second solder resistincludes a portion level with the bottom metal layer. The second solderresist has a second solder resist density. An absolute value of adifference between the first solder resist density and the second solderresist density is lower than about 0.5.

In accordance with yet other embodiments, a package includes a packagesubstrate, which includes a core. The core includes a dielectric layer,and conductive pipes penetrating through the dielectric layer. Thepackage substrate further includes a first metal layer over the core,wherein the first metal layer has a first metal density, a top metallayer overlying the first metal layer, wherein the top metal layer has asecond metal density, a second metal layer under the core, wherein thesecond metal layer has a third metal density, and a bottom metal layerunderlying the second metal layer. The bottom metal layer has a fourthmetal density. A first sum of the first metal density and the secondmetal density minus a second sum of the third metal density and thefourth metal density have an absolute value smaller than about 0.1. Thepackage further includes a first solder resist comprising a portionlevel with the top metal layer, wherein the first solder resist has afirst solder resist density, and a second solder resist comprising aportion level with the top metal layer. The second solder resist has asecond solder resist density. An absolute value of a difference betweenthe first solder resist and the second solder resist is lower than about0.5.

Although the embodiments and their advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of the embodiments as defined by the appended claims. Moreover,the scope of the present application is not intended to be limited tothe particular embodiments of the process, machine, manufacture, andcomposition of matter, means, methods and steps described in thespecification. As one of ordinary skill in the art will readilyappreciate from the disclosure, processes, machines, manufacture,compositions of matter, means, methods, or steps, presently existing orlater to be developed, that perform substantially the same function orachieve substantially the same result as the corresponding embodimentsdescribed herein may be utilized according to the disclosure.Accordingly, the appended claims are intended to include within theirscope such processes, machines, manufacture, compositions of matter,means, methods, or steps. In addition, each claim constitutes a separateembodiment, and the combination of various claims and embodiments arewithin the scope of the disclosure.

What is claimed is:
 1. A package comprising: a package substratecomprising: a middle layer selected from the group consisting of a coreand a middle metal layer; a first plurality of metal layers overlyingthe middle layer, wherein the first plurality of metal layers has afirst total metal density that is equal to a sum of metal densities ofthe first plurality of metal layers; and a second plurality of metallayers underlying the middle layer, with a first total count of thefirst plurality of metal layers equal to a second total count of thesecond plurality of metal layers, wherein the second plurality of metallayers has a second total metal density that is equal to a sum of metaldensities of the second plurality of metal layers, and wherein anabsolute value of a difference between the first total metal density andthe second total metal density is lower than about 0.1.
 2. The packageof claim 1, wherein the middle layer is a core layer comprising; adielectric layer; and conductive pipes penetrating through thedielectric layer, wherein the first plurality of metal layers iselectrically coupled to the second plurality of metal layers through theconductive pipes.
 3. The package of claim 1, wherein the middle layercomprises a metal layer comprising metal traces, wherein the firstplurality of metal layers is electrically coupled to the secondplurality of metal layers through the metal traces.
 4. The package ofclaim 1 further comprising: a first solder resist comprising a firstportion coplanar with a top metal layer of the first plurality of metallayers, wherein the first solder resist has a first solder resistdensity; and a second solder resist comprising a first portion levelwith a bottom metal layer of the second plurality of metal layers,wherein the second solder resist has a second solder resist density, andwherein an absolute value of a difference between the first solderresist density and the second solder resist density is lower than about0.5.
 5. The package of claim 4, wherein the first solder resist furthercomprises second portions covering portions of first metal features inthe top metal layer, and wherein the second solder resist furthercomprises second portions overlapped by portions of second metalfeatures in the bottom metal layer.
 6. The package of claim 4 furthercomprising: a device die; and solder regions bonding metal traces in thetop metal layer to the device die through Bump-On-Trace (BOT) bonding,wherein each of the solder regions contacts a top surface and oppositesidewalls of one of the metal traces in the top metal layer.
 7. Thepackage of claim 4 further comprising: a first plurality of solderregions contacting metal features in the top metal layer; and a secondplurality of solder regions contacting additional metal features in thebottom metal layer.
 8. A package comprising: a package substratecomprising: a core comprising conductive pipes; a top metal layeroverlying the core; a bottom metal layer underlying the core, whereinmetal features in the bottom metal layer is electrically coupled tometal features in the top metal layer through the conductive pipes; afirst dielectric layer comprising a first portion level with the topmetal layer, wherein the first dielectric layer has a first density; anda second dielectric layer comprising a first portion level with thebottom metal layer, wherein the second dielectric layer has a seconddensity, and wherein an absolute value of a difference between the firstdensity and the second density is lower than about 0.5.
 9. The packageof claim 8 further comprising: a first plurality of solder regionscontacting metal features in the top metal layer; and a second pluralityof solder regions contacting additional metal features in the bottommetal layer.
 10. The package of claim 8, wherein all metal layersoverlying the core have a first total metal density equal to a sum ofall densities of all metal layers over the core, and wherein all metallayers underlying the core have a second total metal density equal to asum of all densities of all metal layers under the core, and wherein anabsolute value of a difference between the first total metal density andthe second total metal density is lower than about 0.1.
 11. The packageof claim 10, wherein a total count of the all metal layers over the coreis equal to a total count of the all metal layers under the core. 12.The package of claim 8, wherein the first dielectric layer furthercomprises second portions covering portions of metal features in the topmetal layer, and wherein the second dielectric layer further comprisessecond portions overlapped by portions of metal features in the bottommetal layer.
 13. The package of claim 8 further comprising: a devicedie; and solder regions bonding metal traces in the top metal layer tothe device die through Bump-On-Trace (BOT) bonding, wherein each of thesolder regions contacts a top surface and opposite sidewalls of one ofthe metal traces in the top metal layer.
 14. The package of claim 8,wherein one of the first dielectric layer and the second dielectriclayer comprises an epoxy resin, a hardener, a color, a filler, and asolvent, wherein the epoxy resin comprises o-Cresol novolac epoxy resin,phenol epoxy resin, or Diglycidylether Bisphenol A (DGEBA) epoxy resin,wherein an active ingredient of the hardener comprises an reactionproduct of an amine compound selected from imidazole and melamine, andwherein the solvent comprises glycol.
 15. The package of claim 8,wherein a bottom surface of the first dielectric layer is coplanar witha bottom surface of the top metal layer, and wherein a top surface ofthe second dielectric layer is coplanar with a top surface of the bottommetal layer.
 16. A package comprising: a package substrate comprising: afirst plurality of solder regions; a second plurality of solder regions;a core between the first plurality of solder regions and the secondplurality of solder regions, wherein the core comprises: a dielectriclayer; conductive pipes penetrating through the dielectric layer; a topmetal layer overlying the core and comprising first metal features, withthe first metal features in contact with the first plurality of solderregions, wherein all metal layers between the first plurality of solderregions and the core have a first total metal density; and a bottommetal layer underlying the core and comprising second metal features,with the second metal features in contact with the second plurality ofsolder regions, wherein all metal layers between the second plurality ofsolder regions and the core have a second total metal density, andwherein an absolute value of a difference between the first total metaldensity and the second total metal density is lower than about 0.1. 17.The package of claim 16, wherein the all metal layers between the firstplurality of solder regions and the core have a first total count, andthe all metal layers between the second plurality of solder regions andthe core have a second total count equal to the first total count. 18.The package of claim 16 further comprising a device die bonded to thepackage substrate through the first plurality of solder regions.
 19. Thepackage of claim 16 further comprising solder balls attached to metalpads in the bottom metal layer.
 20. The package of claim 16 furthercomprising: a first dielectric layer comprising a first portion levelwith the top metal layer, and second portions, wherein the firstdielectric layer has a first density; and a second dielectric layercomprising a first portion level with the bottom metal layer, whereinthe second dielectric layer has a second density, and wherein anabsolute value of a difference between the first density and the seconddensity is lower than about 0.5.